/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.ObjectArrays.checkElementsNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.concurrent.LazyInit;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.NavigableSet;
import java.util.SortedSet;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;
import java.util.stream.Collector;
import javax.annotation.Nullable;

/**
 * A {@link NavigableSet} whose contents will never change, with many other important properties
 * detailed at {@link ImmutableCollection}.
 *
 * <p>
 * <b>Warning:</b> as with any sorted collection, you are strongly advised not to use a
 * {@link Comparator} or {@link Comparable} type whose comparison behavior is <i>inconsistent with
 * equals</i>. That is, {@code a.compareTo(b)} or {@code comparator.compare(a, b)} should equal zero
 * <i>if and only if</i> {@code a.equals(b)}. If this advice is not followed, the resulting
 * collection will not correctly obey its specification.
 *
 * <p>
 * See the Guava User Guide article on
 * <a href= "https://github.com/google/guava/wiki/ImmutableCollectionsExplained"> immutable
 * collections</a>.
 *
 * @author Jared Levy
 * @author Louis Wasserman
 * @since 2.0 (implements {@code NavigableSet} since 12.0)
 */
// TODO(benyu): benchmark and optimize all creation paths, which are a mess now
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableSortedSet<E> extends ImmutableSortedSetFauxverideShim<E>
        implements NavigableSet<E>, SortedIterable<E> {
    static final int SPLITERATOR_CHARACTERISTICS = ImmutableSet.SPLITERATOR_CHARACTERISTICS | Spliterator.SORTED;

    /**
     * Returns a {@code Collector} that accumulates the input elements into a new
     * {@code ImmutableSortedSet}, ordered by the specified comparator.
     *
     * <p>
     * If the elements contain duplicates (according to the comparator), only the first duplicate in
     * encounter order will appear in the result.
     *
     * @since 21.0
     */
    @Beta
    public static <E> Collector<E, ?, ImmutableSortedSet<E>> toImmutableSortedSet(Comparator<? super E> comparator) {
        return CollectCollectors.toImmutableSortedSet(comparator);
    }

    static <E> RegularImmutableSortedSet<E> emptySet(Comparator<? super E> comparator) {
        if (Ordering.natural().equals(comparator)) {
            return (RegularImmutableSortedSet<E>) RegularImmutableSortedSet.NATURAL_EMPTY_SET;
        } else {
            return new RegularImmutableSortedSet<E>(ImmutableList.<E>of(), comparator);
        }
    }

    /**
     * Returns the empty immutable sorted set.
     */
    public static <E> ImmutableSortedSet<E> of() {
        return (ImmutableSortedSet<E>) RegularImmutableSortedSet.NATURAL_EMPTY_SET;
    }

    /**
     * Returns an immutable sorted set containing a single element.
     */
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E element) {
        return new RegularImmutableSortedSet<E>(ImmutableList.of(element), Ordering.natural());
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@link Comparable#compareTo},
     * only the first one specified is included.
     *
     * @throws NullPointerException if any element is null
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2) {
        return construct(Ordering.natural(), 2, e1, e2);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@link Comparable#compareTo},
     * only the first one specified is included.
     *
     * @throws NullPointerException if any element is null
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2, E e3) {
        return construct(Ordering.natural(), 3, e1, e2, e3);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@link Comparable#compareTo},
     * only the first one specified is included.
     *
     * @throws NullPointerException if any element is null
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2, E e3, E e4) {
        return construct(Ordering.natural(), 4, e1, e2, e3, e4);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@link Comparable#compareTo},
     * only the first one specified is included.
     *
     * @throws NullPointerException if any element is null
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2, E e3, E e4, E e5) {
        return construct(Ordering.natural(), 5, e1, e2, e3, e4, e5);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@link Comparable#compareTo},
     * only the first one specified is included.
     *
     * @throws NullPointerException if any element is null
     * @since 3.0 (source-compatible since 2.0)
     */
    @SuppressWarnings("unchecked")
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2, E e3, E e4, E e5, E e6,
            E... remaining) {
        Comparable[] contents = new Comparable[6 + remaining.length];
        contents[0] = e1;
        contents[1] = e2;
        contents[2] = e3;
        contents[3] = e4;
        contents[4] = e5;
        contents[5] = e6;
        System.arraycopy(remaining, 0, contents, 6, remaining.length);
        return construct(Ordering.natural(), contents.length, (E[]) contents);
    }

    // TODO(kevinb): Consider factory methods that reject duplicates

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@link Comparable#compareTo},
     * only the first one specified is included.
     *
     * @throws NullPointerException if any of {@code elements} is null
     * @since 3.0
     */
    public static <E extends Comparable<? super E>> ImmutableSortedSet<E> copyOf(E[] elements) {
        return construct(Ordering.natural(), elements.length, elements.clone());
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@code compareTo()}, only the
     * first one specified is included. To create a copy of a {@code SortedSet} that preserves the
     * comparator, call {@link #copyOfSorted} instead. This method iterates over {@code elements} at
     * most once.
     *
     * <p>
     * Note that if {@code s} is a {@code Set<String>}, then {@code
     * ImmutableSortedSet.copyOf(s)} returns an {@code ImmutableSortedSet<String>} containing each
     * of the strings in {@code s}, while {@code
     * ImmutableSortedSet.of(s)} returns an {@code
     * ImmutableSortedSet<Set<String>>} containing one element (the given set itself).
     *
     * <p>
     * Despite the method name, this method attempts to avoid actually copying the data when it is
     * safe to do so. The exact circumstances under which a copy will or will not be performed are
     * undocumented and subject to change.
     *
     * <p>
     * This method is not type-safe, as it may be called on elements that are not mutually
     * comparable.
     *
     * @throws ClassCastException if the elements are not mutually comparable
     * @throws NullPointerException if any of {@code elements} is null
     */
    public static <E> ImmutableSortedSet<E> copyOf(Iterable<? extends E> elements) {
        // Hack around E not being a subtype of Comparable.
        // Unsafe, see ImmutableSortedSetFauxverideShim.
        @SuppressWarnings("unchecked")
        Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
        return copyOf(naturalOrder, elements);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@code compareTo()}, only the
     * first one specified is included. To create a copy of a {@code SortedSet} that preserves the
     * comparator, call {@link #copyOfSorted} instead. This method iterates over {@code elements} at
     * most once.
     *
     * <p>
     * Note that if {@code s} is a {@code Set<String>}, then {@code ImmutableSortedSet.copyOf(s)}
     * returns an {@code ImmutableSortedSet<String>} containing each of the strings in {@code s},
     * while {@code ImmutableSortedSet.of(s)} returns an {@code ImmutableSortedSet<Set<String>>}
     * containing one element (the given set itself).
     *
     * <p>
     * <b>Note:</b> Despite what the method name suggests, if {@code elements} is an
     * {@code ImmutableSortedSet}, it may be returned instead of a copy.
     *
     * <p>
     * This method is not type-safe, as it may be called on elements that are not mutually
     * comparable.
     *
     * <p>
     * This method is safe to use even when {@code elements} is a synchronized or concurrent
     * collection that is currently being modified by another thread.
     *
     * @throws ClassCastException if the elements are not mutually comparable
     * @throws NullPointerException if any of {@code elements} is null
     * @since 7.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableSortedSet<E> copyOf(Collection<? extends E> elements) {
        // Hack around E not being a subtype of Comparable.
        // Unsafe, see ImmutableSortedSetFauxverideShim.
        @SuppressWarnings("unchecked")
        Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
        return copyOf(naturalOrder, elements);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by their natural
     * ordering. When multiple elements are equivalent according to {@code compareTo()}, only the
     * first one specified is included.
     *
     * <p>
     * This method is not type-safe, as it may be called on elements that are not mutually
     * comparable.
     *
     * @throws ClassCastException if the elements are not mutually comparable
     * @throws NullPointerException if any of {@code elements} is null
     */
    public static <E> ImmutableSortedSet<E> copyOf(Iterator<? extends E> elements) {
        // Hack around E not being a subtype of Comparable.
        // Unsafe, see ImmutableSortedSetFauxverideShim.
        @SuppressWarnings("unchecked")
        Ordering<E> naturalOrder = (Ordering<E>) Ordering.<Comparable>natural();
        return copyOf(naturalOrder, elements);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by the given
     * {@code Comparator}. When multiple elements are equivalent according to {@code compareTo()},
     * only the first one specified is included.
     *
     * @throws NullPointerException if {@code comparator} or any of {@code elements} is null
     */
    public static <E> ImmutableSortedSet<E> copyOf(Comparator<? super E> comparator, Iterator<? extends E> elements) {
        return new Builder<E>(comparator).addAll(elements).build();
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by the given
     * {@code Comparator}. When multiple elements are equivalent according to {@code compare()},
     * only the first one specified is included. This method iterates over {@code elements} at most
     * once.
     *
     * <p>
     * Despite the method name, this method attempts to avoid actually copying the data when it is
     * safe to do so. The exact circumstances under which a copy will or will not be performed are
     * undocumented and subject to change.
     *
     * @throws NullPointerException if {@code comparator} or any of {@code
     *         elements} is null
     */
    public static <E> ImmutableSortedSet<E> copyOf(Comparator<? super E> comparator, Iterable<? extends E> elements) {
        checkNotNull(comparator);
        boolean hasSameComparator = SortedIterables.hasSameComparator(comparator, elements);

        if (hasSameComparator && (elements instanceof ImmutableSortedSet)) {
            @SuppressWarnings("unchecked")
            ImmutableSortedSet<E> original = (ImmutableSortedSet<E>) elements;
            if (!original.isPartialView()) {
                return original;
            }
        }
        @SuppressWarnings("unchecked") // elements only contains E's; it's safe.
        E[] array = (E[]) Iterables.toArray(elements);
        return construct(comparator, array.length, array);
    }

    /**
     * Returns an immutable sorted set containing the given elements sorted by the given
     * {@code Comparator}. When multiple elements are equivalent according to {@code compareTo()},
     * only the first one specified is included.
     *
     * <p>
     * Despite the method name, this method attempts to avoid actually copying the data when it is
     * safe to do so. The exact circumstances under which a copy will or will not be performed are
     * undocumented and subject to change.
     *
     * <p>
     * This method is safe to use even when {@code elements} is a synchronized or concurrent
     * collection that is currently being modified by another thread.
     *
     * @throws NullPointerException if {@code comparator} or any of {@code elements} is null
     * @since 7.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableSortedSet<E> copyOf(Comparator<? super E> comparator, Collection<? extends E> elements) {
        return copyOf(comparator, (Iterable<? extends E>) elements);
    }

    /**
     * Returns an immutable sorted set containing the elements of a sorted set, sorted by the same
     * {@code Comparator}. That behavior differs from {@link #copyOf(Iterable)}, which always uses
     * the natural ordering of the elements.
     *
     * <p>
     * Despite the method name, this method attempts to avoid actually copying the data when it is
     * safe to do so. The exact circumstances under which a copy will or will not be performed are
     * undocumented and subject to change.
     *
     * <p>
     * This method is safe to use even when {@code sortedSet} is a synchronized or concurrent
     * collection that is currently being modified by another thread.
     *
     * @throws NullPointerException if {@code sortedSet} or any of its elements is null
     */
    public static <E> ImmutableSortedSet<E> copyOfSorted(SortedSet<E> sortedSet) {
        Comparator<? super E> comparator = SortedIterables.comparator(sortedSet);
        ImmutableList<E> list = ImmutableList.copyOf(sortedSet);
        if (list.isEmpty()) {
            return emptySet(comparator);
        } else {
            return new RegularImmutableSortedSet<E>(list, comparator);
        }
    }

    /**
     * Constructs an {@code ImmutableSortedSet} from the first {@code n} elements of
     * {@code contents}. If {@code k} is the size of the returned {@code ImmutableSortedSet}, then
     * the sorted unique elements are in the first {@code k} positions of {@code contents}, and
     * {@code contents[i] == null} for {@code k <= i < n}.
     *
     * <p>
     * If {@code k == contents.length}, then {@code contents} may no longer be safe for
     * modification.
     *
     * @throws NullPointerException if any of the first {@code n} elements of {@code contents} is
     *         null
     */
    static <E> ImmutableSortedSet<E> construct(Comparator<? super E> comparator, int n, E... contents) {
        if (n == 0) {
            return emptySet(comparator);
        }
        checkElementsNotNull(contents, n);
        Arrays.sort(contents, 0, n, comparator);
        int uniques = 1;
        for (int i = 1; i < n; i++) {
            E cur = contents[i];
            E prev = contents[uniques - 1];
            if (comparator.compare(cur, prev) != 0) {
                contents[uniques++] = cur;
            }
        }
        Arrays.fill(contents, uniques, n, null);
        return new RegularImmutableSortedSet<E>(ImmutableList.<E>asImmutableList(contents, uniques), comparator);
    }

    /**
     * Returns a builder that creates immutable sorted sets with an explicit comparator. If the
     * comparator has a more general type than the set being generated, such as creating a
     * {@code SortedSet<Integer>} with a {@code Comparator<Number>}, use the {@link Builder}
     * constructor instead.
     *
     * @throws NullPointerException if {@code comparator} is null
     */
    public static <E> Builder<E> orderedBy(Comparator<E> comparator) {
        return new Builder<E>(comparator);
    }

    /**
     * Returns a builder that creates immutable sorted sets whose elements are ordered by the
     * reverse of their natural ordering.
     */
    public static <E extends Comparable<?>> Builder<E> reverseOrder() {
        return new Builder<E>(Ordering.natural().reverse());
    }

    /**
     * Returns a builder that creates immutable sorted sets whose elements are ordered by their
     * natural ordering. The sorted sets use {@link Ordering#natural()} as the comparator. This
     * method provides more type-safety than {@link #builder}, as it can be called only for classes
     * that implement {@link Comparable}.
     */
    public static <E extends Comparable<?>> Builder<E> naturalOrder() {
        return new Builder<E>(Ordering.natural());
    }

    /**
     * A builder for creating immutable sorted set instances, especially {@code
     * public static final} sets ("constant sets"), with a given comparator. Example:
     * 
     * <pre>
     * {
     *     &#64;code
     *
     *     public static final ImmutableSortedSet<Number> LUCKY_NUMBERS =
     *             new ImmutableSortedSet.Builder<Number>(ODDS_FIRST_COMPARATOR).addAll(SINGLE_DIGIT_PRIMES).add(42)
     *                     .build();
     * }
     * </pre>
     *
     * <p>
     * Builder instances can be reused; it is safe to call {@link #build} multiple times to build
     * multiple sets in series. Each set is a superset of the set created before it.
     *
     * @since 2.0
     */
    public static final class Builder<E> extends ImmutableSet.Builder<E> {
        private final Comparator<? super E> comparator;

        /**
         * Creates a new builder. The returned builder is equivalent to the builder generated by
         * {@link ImmutableSortedSet#orderedBy}.
         */
        public Builder(Comparator<? super E> comparator) {
            this.comparator = checkNotNull(comparator);
        }

        /**
         * Adds {@code element} to the {@code ImmutableSortedSet}. If the {@code ImmutableSortedSet}
         * already contains {@code element}, then {@code add} has no effect. (only the previously
         * added element is retained).
         *
         * @param element the element to add
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code element} is null
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E element) {
            super.add(element);
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSortedSet}, ignoring
         * duplicate elements (only the first duplicate element is added).
         *
         * @param elements the elements to add
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E... elements) {
            super.add(elements);
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSortedSet}, ignoring
         * duplicate elements (only the first duplicate element is added).
         *
         * @param elements the elements to add to the {@code ImmutableSortedSet}
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterable<? extends E> elements) {
            super.addAll(elements);
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableSortedSet}, ignoring
         * duplicate elements (only the first duplicate element is added).
         *
         * @param elements the elements to add to the {@code ImmutableSortedSet}
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterator<? extends E> elements) {
            super.addAll(elements);
            return this;
        }

        @CanIgnoreReturnValue
        @Override
        Builder<E> combine(ArrayBasedBuilder<E> builder) {
            super.combine(builder);
            return this;
        }

        /**
         * Returns a newly-created {@code ImmutableSortedSet} based on the contents of the
         * {@code Builder} and its comparator.
         */
        @Override
        public ImmutableSortedSet<E> build() {
            @SuppressWarnings("unchecked") // we're careful to put only E's in here
            E[] contentsArray = (E[]) contents;
            ImmutableSortedSet<E> result = construct(comparator, size, contentsArray);
            this.size = result.size(); // we eliminated duplicates in-place in contentsArray
            return result;
        }
    }

    int unsafeCompare(Object a, Object b) {
        return unsafeCompare(comparator, a, b);
    }

    static int unsafeCompare(Comparator<?> comparator, Object a, Object b) {
        // Pretend the comparator can compare anything. If it turns out it can't
        // compare a and b, we should get a CCE on the subsequent line. Only methods
        // that are spec'd to throw CCE should call this.
        @SuppressWarnings("unchecked")
        Comparator<Object> unsafeComparator = (Comparator<Object>) comparator;
        return unsafeComparator.compare(a, b);
    }

    final transient Comparator<? super E> comparator;

    ImmutableSortedSet(Comparator<? super E> comparator) {
        this.comparator = comparator;
    }

    /**
     * Returns the comparator that orders the elements, which is {@link Ordering#natural()} when the
     * natural ordering of the elements is used. Note that its behavior is not consistent with
     * {@link SortedSet#comparator()}, which returns {@code null} to indicate natural ordering.
     */
    @Override
    public Comparator<? super E> comparator() {
        return comparator;
    }

    @Override // needed to unify the iterator() methods in Collection and SortedIterable
    public abstract UnmodifiableIterator<E> iterator();

    /**
     * {@inheritDoc}
     *
     * <p>
     * This method returns a serializable {@code ImmutableSortedSet}.
     *
     * <p>
     * The {@link SortedSet#headSet} documentation states that a subset of a subset throws an
     * {@link IllegalArgumentException} if passed a {@code toElement} greater than an earlier
     * {@code toElement}. However, this method doesn't throw an exception in that situation, but
     * instead keeps the original {@code toElement}.
     */
    @Override
    public ImmutableSortedSet<E> headSet(E toElement) {
        return headSet(toElement, false);
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public ImmutableSortedSet<E> headSet(E toElement, boolean inclusive) {
        return headSetImpl(checkNotNull(toElement), inclusive);
    }

    /**
     * {@inheritDoc}
     *
     * <p>
     * This method returns a serializable {@code ImmutableSortedSet}.
     *
     * <p>
     * The {@link SortedSet#subSet} documentation states that a subset of a subset throws an
     * {@link IllegalArgumentException} if passed a {@code fromElement} smaller than an earlier
     * {@code fromElement}. However, this method doesn't throw an exception in that situation, but
     * instead keeps the original {@code fromElement}. Similarly, this method keeps the original
     * {@code toElement}, instead of throwing an exception, if passed a {@code toElement} greater
     * than an earlier {@code toElement}.
     */
    @Override
    public ImmutableSortedSet<E> subSet(E fromElement, E toElement) {
        return subSet(fromElement, true, toElement, false);
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public ImmutableSortedSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
        checkNotNull(fromElement);
        checkNotNull(toElement);
        checkArgument(comparator.compare(fromElement, toElement) <= 0);
        return subSetImpl(fromElement, fromInclusive, toElement, toInclusive);
    }

    /**
     * {@inheritDoc}
     *
     * <p>
     * This method returns a serializable {@code ImmutableSortedSet}.
     *
     * <p>
     * The {@link SortedSet#tailSet} documentation states that a subset of a subset throws an
     * {@link IllegalArgumentException} if passed a {@code fromElement} smaller than an earlier
     * {@code fromElement}. However, this method doesn't throw an exception in that situation, but
     * instead keeps the original {@code fromElement}.
     */
    @Override
    public ImmutableSortedSet<E> tailSet(E fromElement) {
        return tailSet(fromElement, true);
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public ImmutableSortedSet<E> tailSet(E fromElement, boolean inclusive) {
        return tailSetImpl(checkNotNull(fromElement), inclusive);
    }

    /*
     * These methods perform most headSet, subSet, and tailSet logic, besides parameter validation.
     */
    abstract ImmutableSortedSet<E> headSetImpl(E toElement, boolean inclusive);

    abstract ImmutableSortedSet<E> subSetImpl(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive);

    abstract ImmutableSortedSet<E> tailSetImpl(E fromElement, boolean inclusive);

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public E lower(E e) {
        return Iterators.getNext(headSet(e, false).descendingIterator(), null);
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public E floor(E e) {
        return Iterators.getNext(headSet(e, true).descendingIterator(), null);
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public E ceiling(E e) {
        return Iterables.getFirst(tailSet(e, true), null);
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public E higher(E e) {
        return Iterables.getFirst(tailSet(e, false), null);
    }

    @Override
    public E first() {
        return iterator().next();
    }

    @Override
    public E last() {
        return descendingIterator().next();
    }

    /**
     * Guaranteed to throw an exception and leave the set unmodified.
     *
     * @since 12.0
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @GwtIncompatible // NavigableSet
    @Override
    public final E pollFirst() {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the set unmodified.
     *
     * @since 12.0
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @GwtIncompatible // NavigableSet
    @Override
    public final E pollLast() {
        throw new UnsupportedOperationException();
    }

    @GwtIncompatible // NavigableSet
    @LazyInit
    transient ImmutableSortedSet<E> descendingSet;

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public ImmutableSortedSet<E> descendingSet() {
        // racy single-check idiom
        ImmutableSortedSet<E> result = descendingSet;
        if (result == null) {
            result = descendingSet = createDescendingSet();
            result.descendingSet = this;
        }
        return result;
    }

    @GwtIncompatible // NavigableSet
    ImmutableSortedSet<E> createDescendingSet() {
        return new DescendingImmutableSortedSet<E>(this);
    }

    @Override
    public Spliterator<E> spliterator() {
        return new Spliterators.AbstractSpliterator<E>(size(), SPLITERATOR_CHARACTERISTICS | Spliterator.SIZED) {
            final UnmodifiableIterator<E> iterator = iterator();

            @Override
            public boolean tryAdvance(Consumer<? super E> action) {
                if (iterator.hasNext()) {
                    action.accept(iterator.next());
                    return true;
                } else {
                    return false;
                }
            }

            @Override
            public Comparator<? super E> getComparator() {
                return comparator;
            }
        };
    }

    /**
     * @since 12.0
     */
    @GwtIncompatible // NavigableSet
    @Override
    public abstract UnmodifiableIterator<E> descendingIterator();

    /**
     * Returns the position of an element within the set, or -1 if not present.
     */
    abstract int indexOf(@Nullable Object target);

    /*
     * This class is used to serialize all ImmutableSortedSet instances, regardless of
     * implementation type. It captures their "logical contents" only. This is necessary to ensure
     * that the existence of a particular implementation type is an implementation detail.
     */
    private static class SerializedForm<E> implements Serializable {
        final Comparator<? super E> comparator;
        final Object[] elements;

        public SerializedForm(Comparator<? super E> comparator, Object[] elements) {
            this.comparator = comparator;
            this.elements = elements;
        }

        @SuppressWarnings("unchecked")
        Object readResolve() {
            return new Builder<E>(comparator).add((E[]) elements).build();
        }

        private static final long serialVersionUID = 0;
    }

    private void readObject(ObjectInputStream stream) throws InvalidObjectException {
        throw new InvalidObjectException("Use SerializedForm");
    }

    @Override
    Object writeReplace() {
        return new SerializedForm<E>(comparator, toArray());
    }
}
